U.S. patent application number 14/360896 was filed with the patent office on 2015-07-23 for transparent conductive electrodes comprising merged metal nanowires, their structure design, and method of making such structures.
The applicant listed for this patent is Nuovo Film, Inc.. Invention is credited to Hakfei Poon.
Application Number | 20150208498 14/360896 |
Document ID | / |
Family ID | 51370937 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150208498 |
Kind Code |
A1 |
Poon; Hakfei |
July 23, 2015 |
Transparent conductive electrodes comprising merged metal
nanowires, their structure design, and method of making such
structures
Abstract
The present invention discloses transparent conductive
electrodes comprising merged metal nanowires and the method of
making the same. The merged nanowire junctions have junction depth
less than the combination of the diameters of the individual metal
nanowires.
Inventors: |
Poon; Hakfei; (Mountain
View, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nuovo Film, Inc. |
Jiangsu |
|
CN |
|
|
Family ID: |
51370937 |
Appl. No.: |
14/360896 |
Filed: |
January 22, 2014 |
PCT Filed: |
January 22, 2014 |
PCT NO: |
PCT/CN2014/071144 |
371 Date: |
October 20, 2014 |
Current U.S.
Class: |
361/748 ;
174/253 |
Current CPC
Class: |
H05K 1/092 20130101;
H05K 2201/0108 20130101; H05K 1/0306 20130101; B05D 5/12 20130101;
H01L 31/1884 20130101; G02F 2203/01 20130101; B05D 3/0254 20130101;
G02F 2201/12 20130101; G02F 2202/36 20130101; H05K 2201/026
20130101; B05D 1/12 20130101; H01L 31/022466 20130101 |
International
Class: |
H05K 1/02 20060101
H05K001/02; H05K 1/03 20060101 H05K001/03; H05K 1/09 20060101
H05K001/09 |
Claims
1. A conductive electrode, comprising a substrate; and a
substantial single layer on the substrate, comprising a first metal
nanowire, having a diameter of d1, and a second metal nanowire,
having a diameter of d2, wherein the first and second metal
nanowires meet to form a welded junction, having a depth of J12,
wherein J12=<(d1+d2), J12>d1, and J12>d2.
2. The electrode of claim 1, wherein the metal nanowires are silver
nanowires.
3. The electrode of claim 1, wherein the metal nanowires having an
aspect ration more than 1000.
4. The electrode of claim 1, wherein the metal nanowires having a
diameter less than 50 nm.
5. The electrode of claim 1, wherein the conductive layer includes
a matrix.
6. The electrode of claim 1, wherein the matrix material is an
inorganic material.
7. The electrode of claim 1, wherein the nanowire is surface
functionalized.
8. The electrode of claim 1, wherein the electrode has a sheet
resistance of 1-1000 Ohm/sq
9. The electrode of claim 1, having a light transmission of at
least 80%.
10. The electrode of claim 1, having a haze that is tunable from
0.1-10.0.
8. The electrode of claim 1, wherein the substrate is glass.
9. The electrode of claim 1, wherein the substrate is flexible.
10. The electrode of claim 1, further comprising one or more
anti-reflective layers, anti-glare layers, adhesive layers,
barriers, hard coat, or a protective film.
11. A display device comprising at least one transparent electrode
having a conductive layer, the conductive layer including a
plurality of metal nanowires, wherein the metal nanowires intersect
and form a metal nanowire network, said network comprising nanowire
junctions having a depth equal or less than the combination of the
diameters of the individual metal nanowires.
12. The display device of claim 11, wherein the display device is a
touch screen, a liquid crystal display, or a flat panel
display.
13. The display device of claim 1, wherein the metal is silver.
Description
TECHNICAL FIELD OF THE DISCLOSURE
[0001] This present patent application relates, in general, to the
art of transparent electrodes, including their structures and
method of making, and more particularly, to the art of fabricating
transparent electrodes having a network of metal nanowires with
merged junctions.
BACKGROUND OF THE DISCLOSURE
[0002] Indium tin-oxide (ITO) is traditionally widely used as a
transparent conductor in transparent electrodes in science and
research community, but it also has well drawbacks in large scale
manufacturing processes. First, in order to make electrodes, ITO is
vacuum deposited onto substrates, and the vacuum deposition process
is expensive and low throughput. Second, in most of applications,
150 nm or thicker of ITO is needed to ensure electrical
performance, but at such thicknesses, ITO films become brittle
making them not feasible for applications requiring large areas or
flexible substrates. Third, to achieve good conductivity and
clarity, ITO films need to be annealed at high temperatures,
preferably over 200 C, thus limiting its application on high
temperature resistant substrates such as glass. Due to the low
softening point of polymers, most polymer based ITO films cannot
withstand the annealing temperatures required for achieving the
high conductivity and transparency at the same time. Therefore as
electro-optical applications expand to more novel and exotic
functionalities, such as 3-dimentional displays and solar cells,
there is an increasing demand to invent alternative transparent
electrodes with better than or comparable optical and electrical
performance of ITO but suitable for large area flexible substrate
and can be manufactured in an inexpensive high through manner.
[0003] Transparent conductive electrodes comprising printable metal
nanowires have been successfully demonstrated as alternatives to be
manufactured at low cost and on a large scale and with excellent
performance including conductivity and transparency.
[0004] However the networked metal nanowires are not like the ITO
films, having uniform conductivity across the entire film. The
electrode having a plurality of metal nanowires, have areas
containing metal nanowires laying on top of each other or crossing
over. Research has found that reducing the metal nanowire junctions
can significant reduce the sheet resistance of the conductive
film.
[0005] Normally, when two nanowires stack together, it results in
an intersection, having a height equal to the combined heights,
i.e. diameters, of the two nanowires. For example, a conductive
metal nanowire network comprises a first metal nanowire, having a
diameter of d1, and a second metal nanowire, having a diameter of
d2, and in the metal nanowire network, the first and second metal
nanowire cross over to form a junction, then the junction height
(J12) equals to d1+d2. FIG. 5 shows another example, a conductive
electrode comprises a plurality of metal nanowires, the networked
metal nanowires have a first metal nanowire with a diameter of d1,
a second metal nanowire with a diameter of d2, and a third metal
nanowire with a diameter of d3. In the metal nanowire network, the
first, second and third metal nanowire cross over to form a
junction, then the junction height J13 equals to the total height
(i.e. diameter) of each metal nanowire, which is J13=d1+d2+d3. In
FIG. 1, the first, second and third metal nanowire all have the
same diameter (d1=d2=d3=d) and the junction height J13 equals to
3d.
[0006] Research has found that high temperature annealing alone is
not effective in melting the metal nanowire junction in order to
reduce the sheet resistance. For example, anneal the dry film at a
process condition 150-200 C, does not change the junction that has
been formed, the sheet resistance of conductive film remains as
high as over 1000 Ohms.
[0007] Approaches that have proven to be useful to change the
nanowire junction is either to glue two wires together with a
conductive polymer, as what has been taught in the art of carbon
nanotubes, or using a high pressure press to flattened the
junctions, as taught by US Publication 20110285019 and U.S. Pat.
No. 8,049,333 in Cambrios patents. In US Publication 20110285019
and U.S. Pat. No. 8,049,333, external macroscopic force such as
high pressure is used to flatten the junction to achieve the
reduction in sheet resistance, in addition to high temperature
annealing. However, the process introduces defects. Because
nanowires are susceptible to damages, including physical
deformation and/or thermal oxidation under high temperature and
high-pressure process. Also the process using external force
pressing the nanowires together is applied to the entire film, not
only to the metal nanowire junction. Given the tiny dimension of
nanowires, it requires very smooth and flat substrate surface to
ensure the applied forces act on the junction. Otherwise, it is
very likely that the nanowire length besides the junction is also
pressed to be deformed or flattened, causing unnecessary stability
issues.
[0008] In view of the foregoing, a better method to connect
nanowires at the cross over points is needed.
[0009] The present invention discloses an improved way to integrate
nanowires at cross points to form merged junctions, in order to
achieve low sheet resistance of a transparent conductive electrode.
The method disclosed herein does not require high temperature, high
pressure, and does not result in deformed metal nanowires.
SUMMARY
[0010] The present invention discloses a transparent conductive
electrode comprising a substrate; and a substantial single layer on
the substrate, comprising a first metal nanowire, having a diameter
of d1, and a second metal nanowire, having a diameter of d2,
wherein the first and second metal nanowires meet to form a merged
junction, having a depth of J12, wherein J12<(d1+d2), J12>d1,
and J12>d2.
[0011] The present invention also discloses a method of making a
transparent conductive electrode, comprising a plurality of metal
nanowires in a network, said network comprises merged metal
nanowire junctions, the method comprising providing a substrate;
and [0012] forming a substantial single layer comprising metal
nanowire network on the substrate; and [0013] forming merged metal
nanowire junctions between neighboring metal nanowires.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Exemplary embodiments of the disclosure will be more clearly
understood from the following detailed description taken in
conjunction with the accompanying drawings in which:
[0015] FIG. 1 diagrammatically illustrates a cross-section view of
a metal nanowire;
[0016] FIG. 2 diagrammatically illustrates a cross-sectional view
of one example of two metal nanowires meets to form a cross
section;
[0017] FIG. 3 diagrammatically illustrates a cross-sectional view
of one example of two metal nanowires meets to form a flattened
cross section in the prior art;
[0018] FIGS. 4a-b diagrammatically illustrates a cross-sectional
view of one example of two metal nanowires meets to form merged
junction in the present invention;
[0019] FIG. 5 is an SEM image of the cross-section view of a
conductive transparent electrode, wherein three metal nanowires lay
on top of one another;
[0020] FIG. 6 shows an SEM image of a conductive layer following a
post-treatment of pressure application, wherein cross points has a
flattened cross section as in the prior art;
[0021] FIG. 7 shows an SEM image of a conductive layer comprising
metal nanowire merged junctions, wherein the depth of the junction
is less than the combination of the two individual diameters.
DETAILED DESCRIPTION OF SELECTED EXAMPLES
[0022] Hereinafter, selected examples of a transparent conductive
electrode will be discussed with reference to the accompanying
drawings. It will be appreciated by those skilled in the art that
the following discussion is for demonstration purposes, and should
not be interpreted as a limitation. Other variances within the
scope of this disclosure are also applicable.
[0023] "Optional" or "optionally" means that the subsequently
described circumstance may or may not occur, so that the
description includes instances where the circumstance occurs and
instances where it does not.
[0024] In the scope of the present invention, in some instances,
"top" means situated at the highest position in a figure or a
stack. "Top view" means what an observer sees looking down at the
top. In some instances, bottom electrode means a device is built
from it whereas a top electrode means an electrode situated on top
of the device stack. Single layer
[0025] In one embodiment of the present invention, the transparent
conductive electrode (TCE) comprises a substrate and a single
conductive layer, comprising nanowires. Optionally, the conductive
layer further comprises a diffused conductive material, for example
ITO. Optionally, the conductive layer further comprises a matrix,
comprising conductive or non-conductive polymers. "Matrix" refers
to a solid-state material into which the metal nanowires are
dispersed or embedded. Portions of the nanowires may protrude from
the matrix material to enable access to the conductive network. The
matrix may be a host for the metal nanowires and provides a
physical form of the conductive layer. The matrix may protect the
metal nanowires from adverse environmental factors, such as
corrosion and abrasion. In addition, the matrix may offer favorable
physical and mechanical properties to the conductive layer. For
example, it can provide adhesion to the substrate. In one example,
the matrix is organic material, which offers a flexible matrix,
compatible with a polymeric substrate. In another example, the
matrix is metal oxide film, which is more compatible with glass
substrate. The matrix may be refractive index matching layer. The
matrix may offer anti-reflection and antiglare property to the
transparent conductive electrode.
[0026] As used herein, "a single layer" or "a substantial single
layer" is generally less than 150 nm, which is about three-nanowire
thickness. More typically, "a single layer" or "a substantial
single layer" is generally less than 100 nm, two-nanowire
thickness. Preferably, "a single layer" or "a substantial single
layer" is generally 50 nm or less, one nanowire thickness. In
various embodiments, the width or diameter of the nanowires are in
the range of 10 to 40 nm, 20 to 40 nm, 5 to 20 nm, 10 to 30 nm, 40
to 60 nm, 50 to 70 nm.
Nanowires
[0027] In accordance with the aspects with the present invention,
nanowires have a cylindrical shaped, having a diameter d and length
L as shown in FIG. 1. The aspect ratios of nanowires are L/d.
Suitable aspect ratios of the nanowires are between 10 to 100, 000.
In one preferred example, the aspect ratios of the nanowires are
more than 1000, in order to provide a transparent conductive film,
because longer and thinner nanowires may enable more efficient
conductive networks while permitting lower overall density of wires
for achieving a higher transparency.
Metal Nanowires
[0028] As know in the art, conductive nanowires include metal
nanowires and non-metallic nanowires. In general, "metal nanowire"
refers to a metallic wire comprising element metal and metal
alloys. Non-metallic nanowires include, for example, carbon
nanotubes (CNTs), conductive polymer fibers and the like.
[0029] In accordance with the aspects of the present invention,
metal nanowires refers to substantially elemental metal and metal
alloys. Optionally, the metal nanowires may have less than 5-10%
(by moles) of metal oxides. Metal oxides may exist in the metal
nanowire shell or core as an impurity or defect in the nanowire
synthesis.
[0030] In accordance with the aspects of the present invention,
metal oxide nanowires refers to the nanowires are substantially
metal oxides. Optionally, metal oxide nanowires may have less than
5-10% (by moles) of elemental metal, due to incomplete oxidation or
any other reasons.
[0031] In accordance with the aspects of the present invention,
hybrid nanowires are metal/metal oxide nanowires, wherein the
nanowires, having both elemental metal and metal oxides as major
components. Metal/metal oxide hybrid nanowires may comprise 40%
(mole %) metal oxide and 60% (mole %) elemental metal. Metal/metal
oxide hybrid nanowires may comprise 60% (mole %) metal oxide and
40% (mole %) elemental metal.
Conductivity of Metal Nanowire
[0032] A single metal nanowire has to extend between two different
electrical terminals to provide an electrically conductive path
from one terminal to terminal. The term "terminal" includes cathode
or anode or any other starting and ending points that are
electrically connected. Generally, the longer the metal nanowire
the longer the conductive pathway, the more conductive the
conductive electrode and lower the sheet resistance. The more metal
nanowires in a given area, the lower the sheet resistance of the
conductive electrode. In order to achieve both highly conductive
electrode and highly transparent film, the metal nanowires are
preferred to be long and thin.
[0033] However, making a conductive film having super long and thin
is not only experimentally challenging, but can lead to brittle
films. In the conductive layer of the electrode of the present
invention, a plurality of metal nanowires in conductive layer forms
a network. In the network, one nanowire can be related to a
neighboring nanowire through entanglement or loosely crossing over.
When a nanowire is related to another nanowire in proximity, a
charge may or may not be able to hop from one nanowire to another.
In the network, one nanowire can be connected to a neighboring
nanowire through crossing over. When one nanowire connects to
another nanowire, a connecting junction is formed and the
conductive pathways provided by both nanowires are
interconnected.
Over Pass Junctions Flattened Junctions vs. Merged Junctions
[0034] FIG. 2 and FIG. 5 list examples of over pass junctions.
FIGS. 3-4b, and FIG. 7 illustrate examples of merged junctions.
FIG. 6 is an SEM image from US Publication 20110285019 illustrating
flattened junctions.
[0035] FIGS. 2-4b schematically illustrate three examples of metal
nanowire connecting junctions. FIG. 2 illustrates a first kind
connecting junction is an over pass junction, wherein the one
nanowire is laid over the other nanowire and there is no space or
matrix material between the two nanowires. The two nanowire forms a
close interface at the junction, but most of the metal nanowires
are substantially separate from each other. FIG. 3 illustrates a
second kind of connecting junction, a flat junction, wherein the
cross point between the two nanowires are flat. FIGS. 4a and 4b
illustrate a third connecting junction, a merged junction, wherein
one nanowire cross over another nanowire, at least some part of the
nanowire is merged into each other.
[0036] The present invention is directed to a conductive electrode,
comprising a substrate and a substantially a single conductive
layer. The conductive layer comprises a plurality of metal
nanowires networked together. The plurality of metal nanowires are
linked to each other at various points to provide a conductive
pathways from one terminal to another. The plurality nanowires
comprises a first nanowire and a second nanowire network together.
In the conductive nanowire network, the first nanowire is related
to the second nanowire. In the conductive nanowire network, the
first nanowire is connected to the second network. When the first
nanowire is connected to a second nanowire, the conductive pathways
are linked, fused, or merged together. The first nanowire has a
diameter of d1. The second nanowire has a diameter of d2. The
height of the junction, which is the distance from the external
boundary of one nanowire to the external boundary of the other
nanowire, is J12. In the network, when the first nanowire is
related to the second nanowire, the value of J12 is larger than the
combination of (d1+d2). In the network, when the first nanowire is
connected or linked with the second nanowire, the value of J12 is
equal to or greater than the diameter of the individual nanowires,
but less than the combination of the diameters of the individual
nanowires (d1+d2).
[0037] US publication 20110285019 and U.S. Pat. No. 8,049,333
taught flat or flattened cross points. The junction or crossing
points are flattened by pressure or high temperature in order to
reduce the sheet resistance of electrode. In accordance with the
aspects of US publication20110285019 and U.S. Pat. No. 8,049,333,
the cross points or junction of two crossing over nanowires have to
pressed by pressure, to physically deform the metal nanowire
macroscopically, to achieve a flat cross point.
[0038] Further, the method taught by US publication 20110285019 and
U.S. Pat. No. 8,049,333, rolling the transparent conductive
electrode under a roller to flatten the junction is subject to the
surface roughness of the substrate. Using an external press to
flatten junction the pressure is counteracted by the surface
roughness of press roll and the substrate and the conformal contact
between two surfaces is hard to control.
[0039] In contrast, the present invention presents an electrode
having low sheet resistance by comprising nanowire junctions having
merged junctions, wherein the merged junctions do not have
deformed/flattened surfaces. Further, one nanowire merged into
another nanowire without the application of pressure.
[0040] The method of making a transparent conductive electrode,
disclosed herein, comprises providing a substrate; and forming a
substantial single layer comprising metal nanowire network on the
substrate, comprising forming merged metal nanowire junctions
between neighboring metal nanowires.
[0041] The method of forming merged metal nanowire junctions
between neighboring metal nanowires comprises inducing liquid phase
sintering of two nanowires at the cylindrical curvature.
[0042] The method of forming merged metal nanowire junctions
between neighboring metal nanowires further comprise carefully
controlling the drying atmosphere, surface tension, and the
capillary pressure at junction curvature by continuous dissolving
and re-precipitation of silver atoms at the nanowire cross
point.
[0043] The method describes herein utilizes inter-particle forces,
which are much more significant, an order magnitude higher, and
effective than macroscopic forces such as high press rolls to
flatten the metal nanowires. Additionally, the microscopic forces
focus action on the intersection/cross over points only and are
completely independent from the substrate curvature or the surface
roughness of the substrate.
[0044] In one embodiment of the present invention, the method step
forming merged nanowire junctions comprises preparing an ink
solution comprising metal nanowires in a first solvent, forming a
metal nanowire network comprising crossing points on the substrate,
removing the first solvent by drying to form a film of nanowires,
placing the nanowire film under the atmosphere saturate with a
second solvent, controlling the continuous dissolving and
re-precipitation process of the metal nanowire at the cross point
and drying the film to form a conductive film. In one example, the
first solvent and second solvent is the same solvent. In another
example, the second solvent is a combination of two solvents.
[0045] In another embodiment of the present invention, the method
step forming merged nanowire junctions comprises preparing an ink
solution comprising metal nanowires in a first solvent, forming a
metal nanowire network comprising crossing points on the substrate,
forming merged metal nanowire junctions by reducing the evaporation
rate of the first solvent at a first temperature, annealing the
film having merged metal nanowire junctions at a second
temperature.
Coating Methods
[0046] As noted herein, the transparent conductors can be
fabricated by, for example, sheet coating, web-coating, printing,
and lamination. Sheet coating is suitable for coating a conductive
layer on any substrate, in particular, rigid substrates.
Web-coating has been employed in the textile and paper industries
for high-speed (high-throughput) coating applications. It is
compatible with the deposition (coating) processes for transparent
conductor fabrication. Web-coating uses conventional equipment and
can be fully automated, which dramatically reduces the cost of
fabricating transparent conductors. In particular, web-coating
produces uniform and reproducible conductive layers on flexible
substrates. Process steps can be run on a fully integrated line or
serially as separate operations. Further details on the wet-coating
techniques and procedures disclosed by US application 20110285019
can also be adopted in the present invention.
[0047] Optionally, the first metal nanowire network comprising
cross points can be deposited onto the substrate by other methods
than wet-coating and the merge junctions can be formed in the
solvent based atmosphere by controlling the dissolving and
re-precipitation process at the crossing over points or
junctions.
Size of Nanowires
[0048] In one aspect of the present invention, in one example, the
metal nanowires in the network, or in the merged junction have
substantially the same diameters. Then the junction height J12 of
the merged junction between the first and second nanowires, having
diameters d1 and d2 respectively, J12<2d1=2d2.
Transparency
[0049] With preferred thicknesses of the substantial single layer
of the networked metal nanowire, the transparent conductive
electrode provides excellent optical transparency. In one example,
the transparent conductive electrode has at least >80% optical
transmittance in the wavelength of 400-1000 nm. In a preferred
example, the transparent conductive electrode has at least >90%
optical transmittance in the wavelength range of 400-1000 nm. In a
more preferred example, the transparent conductive electrode has at
least >95% optical transmittance from wavelengths of 400-1000
nm.
[0050] The haze value of the transparent conductive electrode in
the present invention are tunable from >10% to <0.6%,
depending on the end use application. In one example of the present
invention, the haze of the transparent conductive electrode is
>10%. In another example of the present invention, the haze of
the transparent conductive electrode is <0.6%. In one example,
the super low haze of the film is achieved by tuning the aspect
ratio of the metal nanowires. In another example, the super low
haze is accomplished by employing index matching materials as a
matrix. In still another example, the super low haze is
accomplished by using index matching as a separate layer.
Conductivity
[0051] The transparent conductive electrode in the present
invention is invented for electrical-optical devices. The single
conductive layer design and the merged junction in the networks are
devised to improve the conductivity in both the in-plane and
off-plane direction. As a result, the sheet resistance of the
conductive film is greatly reduced. In one example, the transparent
conductive electrode has an electrical resistance of about 200 ohms
per square or less. In another example, the transparent conductive
electrode has an electrical resistance of about 300 ohms per square
or less. In another embodiment of the present invention, the metal
nanowire network has a sheet resistance tunable from 0.1 Ohm/sq to
1000 Ohm/sq.
Nanowire Chemical Composition
[0052] In the present invention, nanowires may be comprised of one
or more materials selected from a variety of electrically
conductive materials, any noble elements etc. Elements in the
period table that can be used as the chemical composition for metal
nanowires include, but not limited to, copper (Cu), silver (Ag),
gold (Au), aluminum (Al), nickel (Ni), lead (Pd), platinum (Pt) or
combinations thereof. The metals that can be used in the nanowire
network can further include a silver plated copper, a gold plated
silver, or a gold plated copper. The nanowires may also be
comprised of one or more materials, such as but not limited to, Zn,
Mo, Cr, W, Ta, metallic alloys, or the like. In the present
invention, some less preferred examples include nanowires
comprising metal oxides.
[0053] In one example of the present invention, the metal nanowire
network consists of only one chemical composition throughout. In
another example of the present invention, the metal nanowire
network consists of a mixture of chemical compositions. In one
instance, said mixture of chemical compositions includes metals or
metal oxides. In another instance, said mixture of chemical
compositions includes chemical compounds with different electrical
properties, such as electrical conductivity. In another instance,
said mixture of chemical compositions includes chemical compounds
with different optical properties, such as optical transparency or
refractive index.
[0054] In one example of the present invention, the nanowire may
further comprise an anticorrosion coating or anti-reflective
coating.
Shape or Geometry
[0055] In the aforementioned instances, examples or embodiments of
the present invention disclosed herein, the nanowires are described
as having at least an end or a length. This description is used
primarily for the ease of discussion; it should be understood that
any geometric shapes such as rods of different aspect ratios,
dog-bone shapes, round particles, oblong particles, single or
multiple combinations of different geometric shapes, or other
particle configurations capable of forming a metal network may be
used herein.
Substrate
[0056] In one example of the present invention, the substrate is a
rigid substrate. The rigid substrate is a glass. In some instances,
the glass has refractive index of more than 1.5. In some instances,
the glass has a refractive index of more than 1.7.
[0057] In another example of the present invention, the substrate
is a flexible substrate comprising a polymer. Examples of such a
polymer includes, but not limited to, a polyimides (PI),
polyamides, polyetheretherketone (PEEK), Polyethersulfone (PES),
polyetherimide (PEI), polyethylene naphtalate (PEN), Polyester
(PET), related polymers, a metallized plastic, and/or combination
of the above and/or similar materials.
[0058] In a more preferred example, the polymer substrate has
barrier properties. In one instances, the substrate is a piece of
barrier film having oxygen permeation rate less than
10.sup.-2g/m.sup.2/day. In another instance, the substrate is a
piece of barrier film having moisture permeation rate less than
10.sup.-2g/m.sup.2/day. In still another instance, the substrate is
a piece of barrier film having moisture permeation rate less than
10.sup.-6 g/m.sup.2/day.
[0059] In still another example, the substrate is a curved or
flexible substrate.
[0060] In yet another example, the substrate has regular
geometries. Such geometries include the geometries of cell phones,
tablets, TVs, e-books, windows and solar cells. In yet another
example, the substrate has irregular geometries, including stars,
pyramids and spheres etc.
Location of the Electrode in a Device
[0061] The transparent conductive electrode in the present
invention is ultimately used in electrical optical device. Optical
properties such as transparency and electrical properties like
conductivity make the transparent conductive electrode in the
present invention suitable for a wide range of the applications. In
one example, the transparent electrode is a top electrode in a
device. In another example, the electrode is a bottom electrode of
a device. In still another example, the electrode of is an
electrode is of a stacked device.
Method
[0062] In one aspect, the present invention also discloses a method
of making a transparent conductive electrode, comprising a
plurality of metal nanowires in a network, said network comprises
merged metal nanowire junctions. The method comprising [0063]
providing a substrate; and [0064] forming a substantial single
layer comprising metal nanowire network on the substrate; and
[0065] forming merged metal nanowire junctions between neighboring
metal nanowires.
[0066] In one embodiment, the method of forming a substantial
single layer of transparent conductive electrode, comprises [0067]
preparing an ink solution by mixing nanowires in water, in the
presence of a surfactant; [0068] coating the ink solution onto the
substrate to form a coated film; [0069] drying the coated film in
the ambient environment; and
[0070] anneal the film at a temperature between 80-150 C.
[0071] The method further comprises a step of placing the film in
an acidic environment.
[0072] Subsequently, the method further comprises a step of placing
the film in a basic environment.
[0073] In another embodiment, the method of forming a substantial
single layer of transparent conductive electrode, comprises [0074]
preparing an ink solution by mixing nanowires in water, in the
presence of a surfactant; [0075] coating the ink solution onto the
substrate to form a coated film; [0076] placing the coated film in
an acid environment before the removal of the solvent in the coated
film; and [0077] anneal the coated film at a temperature between
80-150 C.
[0078] Preferably, the method further comprises placing the coated
film in a basic environment after the placing the coated film in an
acid environment.
[0079] Acidic environment includes all the chemical environments,
which are able to convert metals from elemental states to their
oxidation states and be soluble in a solvent or a mixture of
solvents. In one example, the metal nanowire in the transparent
electrode is silver, the solvent used for prepare the ink solution
is water and the acid environment comprises acids, including acetic
acid, formic acid and combinations thereof. Optionally, the ink
solution comprises binders such as cellulose. Optionally, the ink
solution comprises an alcohol as the solvent. Optionally, the ink
solution comprises a water and alcohol mixture as the solvent.
Optionally, the acidic environment comprises more than one acids,
at least one acid is an organic acid.
[0080] Basic environment includes all the chemical environments,
which are able to convert metals in their oxidation or salt states
into elemental states. In one example, the metal nanowire in the
transparent electrode is silver, the solvent used for prepare the
ink solution is water, and the basic environment comprises ammonia
and water. Optionally, the ink solution comprises binders such as
cellulose. Optionally, the ink solution comprises an alcohol as the
solvent. Optionally, the ink solution comprises a water and alcohol
mixture as the solvent. Optionally, the basic environment comprises
more than one base, and at least one base is an organic base.
[0081] The present invention also discloses a method of making a
transparent conductive electrode, comprising a plurality of metal
nanowires in a network, said network comprises merged metal
nanowire junctions. The method comprising providing a substrate;
and [0082] forming a substantial single layer comprising metal
nanowire network on the substrate; and [0083] forming merged metal
nanowire junctions between neighboring metal nanowires.
[0084] In another aspect, the present invention discloses a method
of making a transparent conductive electrode, comprising a
plurality of metal nanowires in a network, said network comprises
merged metal nanowire junctions. The method comprising [0085]
providing a substrate; and [0086] forming a substantial single
layer comprising metal nanowire network on the substrate; and
[0087] forming metal nanowire junctions using liquid phase
sintering process.
[0088] The liquid phase sintering process comprises a key step,
solution-re-precipitation step, wherein some elemental metals are
converted into salts and dissolved, and some dissolved metal salts
precipitate out to form metal powders. The liquid sintering process
further comprises sintering the metal powders into the metal
nanowires.
[0089] The liquid phase sintering is a diffusion-controlled
process.
[0090] Optionally, the liquid phase sintering further comprises
rearranging the metal nanowires.
Experimental
Comparative Experiment:
[0091] A formulation of silver nanowire was prepared by mixing 0.3
g of nanowires, 99.6 g of water, 0.1 g of cellulose, and 0.01g of
surfactants. The solution was then spun coated on a PET substrate
at 800 rpm for 30s and let dry in the air under room temperature
for 10 minutes. This was then further heat dried in an oven at 120
C for another 3 minutes. The sheet resistance of as prepared sample
remains >50K Ohm/sq, and the SEM picture of wire-to-wire
intersection is shown in FIG. 5.
Experimental Procedures to Induce Liquid Phase Sintering
[0092] A formulation of silver nanowire was prepared by mixing 0.3
g of nanowires, 99.6 g of water, 0.1 g of cellulose, and 0.01 g of
surfactants. The solution was spun coated on a PET substrate at 800
rpm for 30 s. Instead of drying in air at room temperature, it was
moved into an acidic atmosphere saturated with both mixture of
acetic and formic acids vapor for 30 s-3 minutes, this is then
moved into a basic atmosphere containing ammonia and water vapor
for 5 minutes. This is then followed with a further bake at
temperature of 120 C for 3 minutes. The sheet resistance of the
sample was measured to be .about.100 Ohm/sq. The SEM picture of
wire-to-wire intersection is shown in FIG. 7.
[0093] It will be appreciated by those skilled in the art that the
above discussion is for demonstration purpose; and the examples
discussed above are some of many possible examples. Other
variations are also applicable.
[0094] Any reference in this specification to "one embodiment," "an
embodiment," "example embodiment," etc., means that a particular
feature, structure, or characteristic described in connection with
the embodiment is included in at least one embodiment of the
invention. The appearances of such phrases in various places in the
specification are not necessarily all referring to the same
embodiment. Further, when a particular feature, structure, or
characteristic is described in connection with any embodiment, it
is submitted that it is within the purview of one skilled in the
art to affect such feature, structure, or characteristic in
connection with other ones of the embodiments. Furthermore, for
ease of understanding, certain method procedures may have been
delineated as separate procedures; however, these separately
delineated procedures should not be construed as necessarily order
dependent in their performance. That is, some procedures may be
able to be performed in an alternative ordering, simultaneously,
etc. In addition, exemplary diagrams illustrate various methods in
accordance with embodiments of the present disclosure. Such
exemplary method embodiments are described herein using and can be
applied to corresponding apparatus embodiments, however, the method
embodiments are not intended to be limited thereby.
[0095] Although few embodiments of the present invention have been
illustrated and described, it would be appreciated by those skilled
in the art that changes may be made in these embodiments without
departing from the principles and spirit of the invention. The
foregoing embodiments are therefore to be considered in all
respects illustrative rather than limiting on the invention
described herein. Scope of the invention is thus indicated by the
appended claims rather than by the foregoing description, and all
changes which come within the meaning and range of equivalency of
the claims are intended to be embraced therein. As used in this
disclosure, the term "preferably" is non-exclusive and means
"preferably, but not limited to." Terms in the claims should be
given their broadest interpretation consistent with the general
inventive concept as set forth in this description. For example,
the terms "coupled" and "connect" (and derivations thereof) are
used to connote both direct and indirect connections/couplings. As
another example, "having" and "including", derivatives thereof and
similar transitional terms or phrases are used synonymously with
"comprising" (i.e., all are considered "open ended" terms)--only
the phrases "consisting of" and "consisting essentially of" should
be considered as "close ended". Claims are not intended to be
interpreted under 112 sixth paragraph unless the phrase "means for"
and an associated function appear in a claim and the claim fails to
recite sufficient structure to perform such function.
* * * * *